Science

What happens when coffee roasts

Drying, browning, first crack and development — the chemistry behind roast character, at the depth that changes what a roaster does.

A dense seed, mostly water and carbohydrate

Green coffee is a seed, and a hard one: roughly a tenth water by weight, with the rest largely carbohydrate, protein, lipid, acids and a small amount of caffeine. It smells of hay and tastes of almost nothing. Everything a roasted coffee is — the sweetness, the aromatics, the browning, the fragility — is created during roasting from those precursors. That is why roasting is the largest single transformation in the chain and why so much of a coffee's character can be made or lost in a few minutes.

Drying: water leaves before anything else can happen

The first phase is dominated by moisture leaving the bean. Water evaporating absorbs a great deal of energy, which is why bean temperature climbs slowly at first however much heat is applied, and why the phase cannot simply be rushed. Little of flavour interest is happening yet, but what happens here constrains everything after it: a bean that has not dried evenly cannot brown evenly, and rushing this phase is a common cause of roasts that look finished on the outside and taste underdeveloped.

Browning: where coffee flavour is created

As moisture falls and temperature rises, two families of reaction begin. The Maillard reactions — between amino acids and reducing sugars — generate an enormous number of aromatic compounds and the brown pigments that give roasted coffee its colour. Caramelisation, the thermal breakdown of sugars, runs alongside them. Between them these produce most of what anyone recognises as coffee flavour, and they consume the sugars as they go, which is the tension at the heart of roasting: the reactions that create complexity also consume the sweetness they are creating it from. This is a summary rather than a mechanism. The chemistry is genuinely complicated, hundreds of compounds are involved, and treatments that reduce it to a single tidy reaction are simplifying past the point of usefulness.

First crack

Pressure builds inside the bean from steam and from carbon dioxide generated by the reactions, until the structure fails audibly — a sound like quiet popcorn, and the landmark every roast is timed against. The bean expands, its structure becomes porous and brittle, and chaff separates. From here the coffee is nominally drinkable. What happens next is the roaster's actual decision.

Development: the decision that defines the roast

The stretch after first crack is where a roast is finished or spoiled. Too short and the reactions begun in browning are left incomplete: the coffee tastes sharp, grassy and thin, and no brewing adjustment recovers it. Too long and roast character progressively displaces origin character, acidity flattens and bitterness climbs, until at second crack oils migrate to the surface and the coffee tastes largely of roasting rather than of coffee. There is no correct amount, only a choice about what you want the coffee to be — and light and dark are directions on this axis rather than fixed colours.

Rate matters more than temperature

Two roasts reaching the same final temperature can taste completely different, because what determines the outcome is how heat was applied on the way there. A roast whose temperature climb stalls or reverses through browning produces the flat, bready result roasters call baking, and it cannot be fixed by roasting for longer — the aromatics have been driven off without the sweetness developing. This is why roasters record the shape of a roast rather than only its endpoint, and why colour alone is a poor description of what a roast is.

Cooling, and why it is part of the roast

Beans dropped from a roaster carry enough heat to continue roasting themselves for some minutes. Cooling quickly is therefore not tidying-up but the last act of the roast: the difference between the roast you chose and one appreciably darker. Commercial roasters use forced-air cooling trays for exactly this reason, and a home roaster without one is well served by moving air through the beans immediately.

What roasting does not change much

Two claims circulate widely and deserve care. Roast level's effect on caffeine is far smaller than commonly believed and is complicated by whether coffee is measured by scoop or by weight — beans lose mass as they roast, so a scoop of dark roast contains fewer beans than a scoop of light. And roasting cannot add what the green coffee did not have: a roaster can lose the character of a fine lot but cannot create it in an ordinary one.

Sources

Technical writing on coffee roasting and extraction
The World Atlas of Coffee